1. Field of the Invention
The present invention relates to a metal-sulfur battery, more particularly to a cathode for metal-sulfur batteries.
2. Discussion of Related Art
In accordance with miniaturization, weight reduction, and performance enhancement of electronic products, communication devices, and the like, research into next-generation high-capacity lithium sulfur batteries, which are recently attracting attention, is ongoing.
Such a lithium-sulfur battery is a secondary battery in which a sulfur-based compound having a sulfur-sulfur bond is used as a cathode active material and a carbon-based material, in which alkali metal ions such as lithium ions are intercalated and deintercalated, is used as an anode active material. Such a lithium-sulfur battery stores and generates electrical energy using oxidation-reduction wherein an oxidation number of sulfur is decreased when a bond between sulfur and sulfur is broken during reduction (discharge) and a bond between sulfur and sulfur is formed again when an oxidation number of sulfur increases during oxidation (charge).
FIG. 1 illustrates the structure and reaction mechanism of a conventional lithium sulfur battery.
Referring to FIG. 1, during discharge of the conventional lithium sulfur battery, lithium is oxidized at an anode and sulfur is reduced at a cathode. Here, cyclic S8 of sulfur is converted into linear lithium polysulfide due to reduction. When the lithium polysulfide is completely reduced, lithium sulfide (Li2S) is generated. In this process, the discharge voltage of the lithium sulfur battery is generated.
Sulfur, which is an element used as a cathode active material of such a lithium sulfur battery, has a high energy density compared to the mass thereof, is cheap, and is harmless to the human body. Accordingly, sulfur is known as a very attractive cathode material for secondary batteries.
However, it is difficult to commercialize such a lithium-sulfur battery because, when sulfur, as a nonconductor not having electrical conductivity, is used as an active material, a conductive material for smoothly providing electrochemical reaction sites is required to induce electrochemical reactions. In addition, the lifespan of the lithium sulfur battery is shortened due to high solubility of lithium polysulfide, which is an intermediate product generated during an electrochemical reaction in the lithium sulfur battery, in an organic electrolyte.
Accordingly, to address such problems, research into a method of developing a carbon material, which is a superior conductive material for activating sulfur, and a combination of materials including carbon, or a method of forming a material having a property of adsorbing sulfur in a single film form and adding the same during a battery manufacturing process to delay elution of sulfur is underway.
Korean Patent Application Publication No. KR2012-0135808 discloses a method of preventing elution of polysulfide using a polymer, i.e., polypropylene, treated with polyethylene glycol to form a hydrophilic porous membrane.
However, this method is disadvantageous in that polysulfide elution is not sufficiently prevented when the loading amount of sulfur is increased, and the performance of a battery is also poor.